1. Field of the Invention
The present invention relates generally to methods for detecting defects in devices fabricated on wafer substrates, and more particularly to a defect detection method for devices that are fabricated utilizing repetitive fabrication techniques such as are conducted with photolithographic masks.
2. Description of the Prior Art
Devices such as recording heads are typically fabricated in large numbers upon the surface of a wafer substrate, and following fabrication the wafers are sliced and diced to ultimately yield individual recording head devices. During the fabrication process, photolithographic techniques are employed which utilize a plurality of masks to shield and expose desired areas of the wafer surface for processing, as is well known to those skilled in the art. Such photolithographic masks are used in a repetitive manner to successively expose adjacent areas across the surface of the disk. Therefore, where a defect exists in an element of such a mask, the defect will repetitively inserted into particular devices on the substrate surface as the mask is successively utilized. The use of such a flawed mask, therefore creates a repetitive pattern of defective devices across the surface of the wafer.
Device testing for defects is conducted in the prior art by testing the electrical resistance of devices after they have been fabricated. In this regard, a resistance test range is developed and devices that exhibit a resistance within the range are deemed acceptable, while devices that exhibit a resistance beyond the range are deemed defective. A particular problem with this type of resistance testing is encountered in the testing of devices such as recording heads for tape drives. Such recording heads include a plurality of sensors disposed adjacent one another, and significantly, the resistance of each adjacent sensor typically differs from each other sensor. One reason for the resistance differences between the sensors is that the electrical lead lines of adjacent sensors differ in length. As a result, even though the sensors of a tape head might be virtually identical, the electrical resistance of the sensors will differ to a significant degree owing to the differing electrical lead line lengths of the sensor. As a result, a simple electrical resistance test requires a rather large range in order to not have good sensors be designated as defective because the electrical resistance of such sensors is beyond the acceptable test range. On the other hand, where the acceptable resistance test range is made large, defective sensors may be included within the acceptable test range. A defect range of 5 to 7% has been common in the prior art, where further analysis reveals that many of the devices initially identified as being defective are actually not defective. There is therefore a need for a test method that more accurately determines whether acceptable or defective devices exist on a wafer substrate.